1. Field of the Invention
This invention relates to an optical switch for switching the transmission path of an optical signal by a change in refractive index, and particularly to an optical switch in which adverse effects of heating can be restrained.
2. Description of the Related Art
The current communication networks such as LANs (local area networks) and WANs (wide area networks) usually employ a communication system that transmits information on electrical signals.
A communication method of transmitting information on optical signals is used only in trunk networks for transmitting a large quantity of data and some other networks. These networks use “point-to-point” communications and have not yet developed into communication networks that can be called “photonic networks”.
To realize such a “photonic network”, an “optical router”, an “optical switching hub” and the like that have functions similar to the functions of devices such as a router and a switching hub for switching the destination of an electrical signal are needed.
Such devices need an optical switch for switching the transmission path at a high speed. There are optical switches using ferroelectric materials such as lithium niobate and PLZT (lead lanthanum zirconate titanate), and an optical switch having an optical waveguide formed in a semiconductor, to which carriers are injected to change the refractive index and thus switch the transmission path of an optical signal.
Recently, there also exists an optical switch in which a heater integrated on a flat glass optical waveguide generates heat to change the refractive index at the part where the heater is formed, thus making a switching operation.
The following are references of the related art of the conventional optical switch having an optical waveguide formed in a semiconductor, to which carriers are injected to change the refractive index and thus switch the transmission path of an optical signal:
JP-A-5-165067;
JP-A-6-130236;
JP-A-6-289339; and
Baujun Li, Guozheng Li, Enke Liu, Zuimin Jiang, Chengwen Pei and Xun Wang, Appl. Phys. Lette., pp. 1-3, 75 (1999).
FIGS. 1 and 2 are plan and sectional views showing an example of the conventional optical switch described in Baujun Li, Guozheng Li, Enke Liu, Zuimin Jiang, Chengwen Pei and Xun Wang, Appl. Phys. Lette., pp. 1-3, 75 (1999).
In FIG. 1, an X-shaped optical waveguide is formed in an optical waveguide layer 2 on a substrate 1, and a rectangular electrode 3 is formed at the intersection of the X-shaped optical waveguide. A rectangular electrode 4 is formed near the intersection of the X-shaped optical waveguide and parallel to the electrode 3. The electrode 3 and the electrode 4 form a pair of electrodes for injecting carriers.
FIG. 2 is a sectional view along a line A-A′ in FIG. 1. A p-type optical waveguide layer 6 of SiGe or the like is formed on a p-type substrate 5 of Si or the like. An X-shaped optical waveguide is formed in the optical waveguide layer 6, and contact layers 7 and 8 are formed at the intersection of the X-shaped optical waveguide and near the intersection of the X-shaped optical waveguide, respectively.
An insulator film 11 of SiO2 or the like is formed except on the contact layers 7 and 8. Electrodes 9 and 10 are formed on the contact layers 7 and 8, respectively.
The operation in the conventional example shown in FIGS. 1 and 2 will now be described. When the optical switch is off, no current is supplied to the electrode 3 (electrode 9) and the electrode 4 (electrode 10).
Therefore, the refractive index at the intersection of the X-shaped optical waveguide shown in FIG. 1 does not change. For example, an optical signal incident from an incidence end indicated by “PI01” in FIG. 1 travels straight through the intersection and is emitted from an emission end indicated by “PO01” in FIG. 1.
On the other hand, when the optical switch is on, electrons are injected from the electrode 3 (electrode 9) and holes are injected from the electrode 4 (electrode 10). Therefore, carriers (electrons and holes) are injected to the intersection.
Accordingly, the refractive index at the intersection of the X-shaped optical waveguide shown in FIG. 1 is lowered by a plasma effect. For example, an optical signal incident from the incidence end indicated by “PI01” in FIG. 1 is totally reflected by the low refractive index part generated at the intersection and is emitted from an emission end indicated by “PO02” in FIG. 1.
As a result, by supplying a current to the electrodes and thus injecting carriers (electrons and holes) to the intersection of the X-shaped optical waveguide to control the refractive index at the intersection, it is possible to control the position where the optical signal is emitted, that is, to switch the propagation path of the optical signal.
In the conventional example shown in FIGS. 1 and 2, carriers (electrons and holes) are injected and a part with a lowered refractive index is generated by plasma effect so that the optical signal is reflected. However, the injection of carriers (electrons and holes) generates heat and this heat causes increase in the refractive index in the part surrounding the heating part.
Therefore, the refractive index at the intersection of the X-shaped optical waveguide increases, affecting characteristics such as cross talk (extinction ratio). Moreover, the adverse effect of this heating varies depending on the frequency of switching.